* src/ltlparse/ltlparse.yy: Fix precedence OP_OR < OP_XOR < OP_AND.

* src/ltlast/binop.cc (binop::instance): Order operands for
associative operators, so that e.g. "a xor b" and "b xor a" are
mapped to the same formula.
* src/ltltest/equals.test: Check this.

src/ltlast/binop.cc

@@ -1,4 +1,5 @@
 #include <cassert>
+#include <utility>
 #include "binop.hh"
 #include "visitor.hh"
 
@@ -89,6 +90,23 @@ namespace spot
     binop*
     binop::instance(type op, formula* first, formula* second)
     {
+      // Sort the operands of associative operators, so that for
+      // example the formula instance for 'a xor b' is the same as
+      // that for 'b xor a'.
+      switch (op)
+	{
+	case Xor:
+	case Equiv:
+	  if (second < first)
+	    std::swap(first, second);
+	  break;
+	case Implies:
+	case U:
+	case R:
+	  // Non associative operators.
+	  break;
+	}
+
       pairf pf(first, second);
       pair p(op, pf);
       map::iterator i = instances.find(p);